1. Field of the Invention
The present invention relates to a new process for the preparation of aromatic acids. More particularly, the invention relates to an two-phase improved process for the production of aromatic acids by hydrocarbonylation of aromatic halides in the presence of a water-immiscible organic solvent, a palladium-based catalyst, a tertiary nitrogen-containing organic base, water and carbon monoxide.
2. Background of the Invention
Two recent patents may be considered among the various known methods of preparing aromatic acids. U.S. Pat. No. 4,034,004 issued July 5, 1977 ("the '004 patent") discloses a process for the preparation of carboxylic acids by the reaction of aromatic or aliphatic organic halides with carbon monoxide in an alkaline medium using phosphinic palladium complexes as catalysts. The reaction takes place in a two-phase medium in the presence of a quaternary alkylammonium salt as a phase transfer agent, with the aqueous phase containing an inorganic alkaline base and the quaternary ammonium salt.
The '004 patent states that the disclosed reaction should take place at a temperature of between 50.degree. and 150.degree. C. However, above this temperature, and even above 120.degree. C., carbon monoxide reacts with the alkaline base to produce sodium formate. The formation of this unwanted by-product needlessly uses up the alkaline base and the carbon monoxide which has been introduced into the reaction mixture, thus greatly reducing the space time yield of the process. As used herein, the term "space time yield" means the quantity of aromatic acid produced per unit (e.g., hour) of reaction time and per unit (e.g., liter) of liquid reaction mixture.
Further, Example 2 of the '004 patent demonstrates the inefficiencies encountered when one attempts to recycle the phosphine-complexed palladium catalyst using the disclosed process. During the first recycle, the yield of aromatic acid produced is about 10% less than the yield of the initial reaction using fresh catalyst. Therefore, the process of the '004 patent cannot be extrapolated to more than 4 or 5 successive recycles due to these increasingly poorer yields.
Patent EP 34,292 reveals a process for the preparation of anthranilic acid by combining carbon monoxide, water, a trialkylamine, a catalyst based on palladium complexed with a phosphine, and an acylamino-, iodo- or bromobenzene.
However, the quantity of water employed is so low that a two-phase system is not produced. Rather, the reaction of this patent takes place in a single, highly viscous homogeneous phase. As a result, the process of EP 34,292 does not permit easy separation of the reaction product from the catalyst and does not allow the catalyst to be readily recycled. Therefore, while the disclosed process is suitable for producing pharmaceutical products or chemical intermediates having a high unit value which are produced in relatively small amounts, it is not well-adapted for products which are produced on an industrial scale for large-volume use.
In contrast, the present invention provides improved processes which can be exploited economically on a large, industrial scale.